1. Field of the Invention
The present invention relates to an image forming apparatus such as electrophotographic copiers, printers or the like and image forming method, and more specifically relates to an image forming apparatus and image forming method for changing the recording sheet transport spacing according to environmental conditions.
2. Description of the Related Art
In image forming apparatuses of the electrophotographic type such as copiers, printers and the like, an electrostatic latent image of the image to be reproduced is formed on the surface of a photosensitive member, said latent image is developed as a toner image, and said toner image is transferred onto a recording sheet and subjected to a fixing process to form the finished image on said recording sheet.
In the aforesaid processing, for example, the surface of a photosensitive member is uniformly charged to a negative polarity by a charger beforehand, and the image to be reproduced is projected onto the charged surface of the photosensitive member so as to discharge the area in which the image is projected, and this image is developed by toner charged to a negative polarity such that said charged toner only adheres to the image area and forms a toner image. In order to transfer the toner image formed on the surface of the photosensitive member onto a recording sheet, a charge having an opposite polarity (positive) is applied to the recording sheet via a transfer charger disposed on the underside of said recording sheet so as to transfer said toner image to the recording sheet. At this time, the recording sheet is electrostatically adhered to the photosensitive member because the photosensitive member is charged with a negative polarity and the recording sheet is charged with a positive polarity. Therefore, in order to separate the recording sheet from the photosensitive member, the charge of the recording sheet is neutralized by a seperation charger in a process to separate the recording sheet from the photosensitive member.
When the recording sheet separates from the photosensitive member, the charge of the recording sheet is neutralized to a certain degree although not completely neutralized by the separation charger, such that a residual positive charge remains on the recording sheet and the photosensitive member is charged with a negative polarity. Therefore, when the trailing edge of the recording sheet separates from the photosensitive member, a discharge is generated between the sheet and the photosensitive member in a phenomenon called a separation discharge which causes the position corresponding to the trailing edge of the recording sheet on the surface of the photosensitive member to be charged with a positive polarity. In the present specifications, the trailing edge of the recording sheet is defined as the edge of the recording sheet on the downstream side relative to the recording sheet transport direction in the image forming apparatus.
After the toner image formed on the surface of the photosensitive member has been transferred to the recording sheet, the photosensitive member is subjected to an optical erasure process by passing below an eraser lamp to eliminate the remaining residual charge. In the part of the photosensitive member charged with a positive polarity by the aforesaid separation discharge, however, the residual charge cannot be eliminated by the optical erasure process and travels to the charger with the residual positive polarity charge intact. Although a charge of negative polarity is imparted to the photosensitive member by the charger, a potential difference cannot accumulate in aforesaid area of the residual charge of positive polarity such that a shift occurs in the surrounding charge to the positive polarity side at position corresponding to the trailing edge of the recording sheet on the surface of the photosensitive member, and toner adheres in this area by the developing process for a subsequent image so as to cause black lines (noise) in the developed image.
FIG. 5 illustrates the situation in which black lines (noise) appear in a subsequent image when the photosensitive member has a residual charge of positive polarity due to separation discharge. A toner image formed on the surface of photosensitive member "a" having a diameter D is transferred onto a first recording sheet P1', and when a separation discharge is generated at point S when the trailing edge of recording sheet P1' separates from photosensitive member "a", a residual positive charge is imparted at area S on the surface of photosensitive member "a". An image to be transferred to a second recording sheet P2' is subsequently formed on the surface of photosensitive member "a". At this time, a line-like toner image is formed in the area of residual positive charge when said image is developed by toner due to the positive charge remaining at area S on the surface of photosensitive member "a". When the transport spacing M between the first recording sheet P1' and second recording sheet P2' is shorter than the circumferential length L (=.mu.D) of the photosensitive member "a" (M&lt;L), the line-like toner image adhered by the residual charge imparted by the separation discharge is developed as line "t" on the image transferred to the second recording sheet P2'.
Particularly in systems which form color images, a plurality of image formations are repeated for the separate colors, and the transfer process of transferring the toner on the surface of photosensitive member is also repeated a plurality of times. Therefore, there is an increase in the residual charge on the recording sheet and separation discharge readily occurs.
In color image processing, unlike typical monochrome image processing, colored image is also formed in the area of the background of an image. Therefore, an image is frequently formed in the area of the trailing edge of the recording sheet and separation discharge readily occurs.
Heretofore two measures have been proposed as countermeasures to the previously described image noise induced by separation discharge. A first countermeasure is a method wherein a charge is imparted by a charger to the area of abnormal charge induced by separation discharge so as to return said area to its original potential. Specifically, a charger is disposed on the downstream side of the transfer charger and on the upstream side of a cleaner (i.e., a device which cleans the surface of the photosensitive member by removing residual toner remaining on said surface after a toner image formed on the photosensitive member has been transferred to recording sheet) relative to the direction of rotation of the photosensitive member, and this charger imparts a potential of opposite polarity to the area of the photosensitive member charged by separation discharge to return said area to its original potential, and thereafter said charge is erased by an optical erasure process.
This method, however, requires the provision of a charger to correct the potential of the photosensitive member in addition to the essential construction for a normal electrophotographic process, and is disadvantageous insofar as this method increases the number of components and cost. Notably, in tandem-type image forming apparatuses, the aforesaid method produces deeper disadvantages than increased number of components and cost due to the necessity of multiple preparations. Tandem-type image forming apparatuses include image forming apparatuses provided with a plurality (in this example, four) electrophotographic processing units of a plurality of colors corresponding to, e.g., yellow (Y), magenta (M), cyan (C), black (Bk), to form color images, wherein the toner image of each color formed on the surface of a photosensitive member provided with said electrophotographic units are consecutively transferred to and overlaid on a recording sheet to form a color image.
When the aforesaid charger is disposed downstream from the transfer charger and upstream from a cleaner relative to the direction of rotation of the photosensitive member, the spatial arrangement of the charger cannot be assured without enlarging the diameter of the photosensitive member. On the other hand, when the diameter of the photosensitive member increases, the size of the image forming apparatus also increases. Particularly in the case of tandem type image forming apparatuses, enlarging the diameter of the photosensitive member produces a markedly greater effect on size than in the case of typical image forming apparatuses (of the type which form images using a single processing unit) because each of the photosensitive members of a plurality of electrophotographic processing units must be enlarged. In tandem type image forming apparatuses provided with delay memory to adjust the shifting of image data processing timing among the various electrophotographic processing units, increasing the size of the photosensitive member results in increased distance between said electrophotographic processing units and necessitates a delay memory of greater capacity.
A second countermeasure is to increase the transport spacing between recording sheets. Specifically, if the transport spacing of the recording sheets is lengthened so as to be greater than the circumferential length of the photosensitive member, the abnormally charged area of the photosensitive member charged by separation discharge will not enter the region to which an image to be transferred to a subsequent recording sheet is formed, as can be clearly seen in FIG. 5. When the transport spacing of the recording sheets is increased in this way so as to be greater than the circumferential length of the photosensitive member, the photosensitive member must idle rotate (rotation without image formation) at least one rotation to pass under the charger before image formation to neutralize the abnormal charge. Accordingly, when the charger imparts a charge to the surface of the photosensitive member in the image formation stage, the influence of the separation discharge is eliminated and the surface of the photosensitive member can be uniformly charged.
The following disadvantages arise, however, when the transport spacing of the recording sheets is increased. A first disadvantage is a reduction in production efficiency caused by a reduction in the number of image formations that can be accomplished per unit time. A second disadvantage is the increased cost of a single image formation sheet caused by the reduced service life of the photosensitive members and developing devices due to the increased operating time of said photosensitive members and developing devices compared to the same number of image formations when the transport spacing between recording sheets is not increased.
For example, when the diameter of the photosensitive member is 60 mm and the processing speed is 120 mm/sec, the transport spacing of the recording sheets must be a minimum of 60 mm.times.3.14, or about 190 mm to avoid the effects of separation discharge. In typical image forming apparatuses, a transport spacing of about 60 mm is adequate even considering the scanner return time.
In image forming processes for A4 size portrait oriented (i.e., the lengthwise direction of the sheet is parallel to the transport direction) recording sheets, the typical image forming apparatus produces 120.times.60/(210+60)=27 sheets/min, whereas increasing the transport spacing of the recording sheets to avoid the effect of separation discharge produces 120.times.60/(210+190)=18 sheets/min. That is, even at identical processing speeds, the number of image formations that can be processed per unit time is reduced to about 2/3. Furthermore, the service life of consumable components is also reduced by this same ratio.